Rechargeable lithium ion batteries need to be stable and have a long cycle life. Traditional lithium ion batteries that use carbon as an anode have a cycle life which typically ranges from 500-1000 cycles. Lithium ion batteries that use lithium titanate as anode material (negative electrode) have been shown to exhibit cycle life in excess of 15,000 cycles.
Degradation of the electro-active material from reactions with electrolyte in a rechargeable cell over the course of time leads to diminished cell performance. The objective in the ‘formation step’ in cell manufacturing is to build passive protective layers over the electroactive materials which are designed to minimize impedance in cell operation while providing structural integrity of the electroactive material from attack by the electrolyte over time with cycling.
In the case of carbon anodes, this formation step involves the activated cell undergoing specific patterns of applied voltage, temperature and time conditions in a step-wise manner (formation profile) so that a customized protective Solid Electrolyte Interface (SEI) layer is grown.
In the case of cells with lithium titanate anodes, the SEI layer is built over the cathode electroactive material (positive electrode) and is generated by unique specific conditions of voltage, temperature and time. The synthesis of these special layers through a specific formation profile leads to the growth of the SEI layer on the cathode.
Byproducts, such as gases, are formed as a result of the reaction delineated in the formation profile. Special processing techniques have to be used in the execution of the formation profile conditions to eliminate these by products from the packaged cell.
The presence of the water in traces amounts in lithium ion cells may have strong detrimental effects on cycle and calendar life of the cell. Moisture in the cell can decrease capacity retention, increase cell impedance and cause intensive cell gassing. Water can be introduced into the cell system in a variety of ways. Some water may be contained within the components of the cell. Some water may be introduced during processing. Some water may actually be formed as a result of reactions taking place with available protons and hydroxyl radicals. Steps can be taken to reduce, control and eliminate the water content within the final sealed cell components so as to increasing cycle life and cell performance Advances in Lithium Ion Batteries, Edited by Walter van Schakwijk and Bruno Scrosati, Table 1, p. 20-21 and p. 163, T. Kawamura et al., Journal of Power Sources 156 (2006) 547-554 and T. Kawamura et al., Extended Abstracts of 41 Battery Symposiums in Japan, p. 280, 2000.
There is a need to extend the useful life of electrodes in a rechargeable lithium ion cell by using improved formation conditions and protocols that can significantly decrease or completely eliminate the moisture content in the cell. The present invention provides methods for decreasing or eliminating water within the system after cell activation.